1. Field of the Invention
The present invention relates to an emulsion polymerization method for preparing carboxylated styrene-butadiene latex which can be used for paper coating, using tertiary-dodecyl mercaptan as chain transfer agent to control the reaction for desired gel content.
2. Description of Prior Art
The emulsion polymerization method for latex can be classfied into three ways; one is to classify low temperature polymerization method and high temperature polymerization method according to the temperature; another is to classify batch charge method and continuous charge method according to the charge method; the third is seeded polymerization method and continuous polymerization method according to whether it uses seed latex or not. However, whichever method may be selected, the batch charge has to be used at the initial polymerization step. In other words, batch charge has to be included at the initial polymerization step even at the continuous charge method.
In the case of emulsion polymerization method for preparing styrene-butadiene latex using tertiary dodecyl mercaptan as chain transfer agent, there is one critical problem for controlling the gel content. It comes from the fact that the decomposition or the reaction rate of tertiary-dodecyl mercaptan is much faster than the conversion rates of the monomers including butadiene. After the total exhaustion of tertiary-dodecyl mercaptan, therefore, unreacted double bonds of butadiene allow the sites for cross-linking to result in rapid increase of the gel content.
Generally, in the system which includes butadiene monomer more than 20 wt %, the latex having lower gel content than 40 wt % can be prepared either by the low temperature polymerization method, or by suspending the polymerization at the desired polymerization degree. On the contrary, the latex having higher gel content than 50 wt % can be obtained through the high tempertature polymerization method, but the reaction should be controlled for desired gel content after exhaustion of tertiary-dodecyl mercaptan. Recently, methylstyrene dimer as chain trasfer agent, solely or mixed with tertiary-dodecyl mercaptan, has been used for obtaining such latex.
To solve above mentioned problems, the inventors have researched and accomplished a novel polymerization method for controlling the desired gel content (50.about.90%) by continuously charging tertiary-dodecyl mercaptan in increment polymerization step without rapid increase of gel content and molecular weight.